A more accurate means of diagnosing PPID is through the use of the TRH stimulation test to compensate for fluctuations in ACTH concentrations due to season, stress, exercise and disease. In general, baseline ACTH has an overall accuracy >90% with median sensitivity >75%. In autumn months, these parameters diminish and confound diagnosis.
A study looked at 20 adult horses segregated into specific groups and season tested: six controls and six horses with PPID tested in autumn; eight controls and six horses with PPID tested in winter; and three controls and three horses with PPID tested in both seasons [Kam, Y.N.; McKenzie, K.; Coyle, M.; Bertin, F.R. Repeatability of a thyrotropin-releasing hormone stimulation test for diagnosis of pituitary pars intermedia dysfunction in mature horses. Journal of Veterinary Internal Medicine Oct 2021; DOI: 10.1111/jvim.16281].
Testing on two occasions was done one week before and one week after the winter solstice and similarly around the autumn equinox.
A baseline blood sample was obtained, and another was taken 30 minutes after IV injection of 1 mg of TRH. Samples were sent to the lab for ACTH values.
- In winter, the TRH stimulation test had an excellent test-retest reliability.
- In autumn, ACTH concentrations following TRH injection were lower on the second test, yet the test is considered to have a “good” test-retest reliability.
In conclusion, when performed two weeks apart around the winter solstice, the TRH stimulation test was repeatable. During the winter, the hypothalamic-pituitary axis is in a more quiescent phase with less variability in ACTH concentrations. However, in autumn months, testing around the autumn equinox results in greater variability in ACTH concentrations taken 30 minutes after TRH injection—this indicates that there is less test repeatability during that season.
Based on these findings, the authors reported: “Repeat testing in autumn might be less reliable when monitoring disease progression or response to treatment.”